Write an equation of total energy of a satellite. Why is the total energy of a satellite negative?

(N/A) The total energy $(E)$ of a satellite of mass $(m)$ orbiting a planet of mass $(M)$ at a distance $(r)$ from the center is the sum of its kinetic energy $(K)$ and potential energy $(U)$.
$K = \frac{GMm}{2r}$
$U = -\frac{GMm}{r}$
$E = K + U = \frac{GMm}{2r} - \frac{GMm}{r} = -\frac{GMm}{2r}$
The total energy is negative because the satellite is in a 'bound state' within the gravitational field of the planet. $A$ negative total energy indicates that the satellite does not have sufficient energy to escape the gravitational pull of the planet to reach infinity. To make the total energy zero (the condition for escape),an external energy equal to $+\frac{GMm}{2r}$ must be supplied to the satellite.